/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.solr.search.function;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.search.Searcher;

import java.io.IOException;
import java.util.Map;

/**
 * <code>ReciprocalFloatFunction</code> implements a reciprocal function f(x) =
 * a/(mx+b), based on the float value of a field or function as exported by
 * {@link org.apache.solr.search.function.ValueSource}. <br>
 * 
 * When a and b are equal, and x>=0, this function has a maximum value of 1 that
 * drops as x increases. Increasing the value of a and b together results in a
 * movement of the entire function to a flatter part of the curve.
 * <p>
 * These properties make this an idea function for boosting more recent
 * documents.
 * <p>
 * Example:<code>  recip(ms(NOW,mydatefield),3.16e-11,1,1)</code>
 * <p>
 * A multiplier of 3.16e-11 changes the units from milliseconds to years (since
 * there are about 3.16e10 milliseconds per year). Thus, a very recent date will
 * yield a value close to 1/(0+1) or 1, a date a year in the past will get a
 * multiplier of about 1/(1+1) or 1/2, and date two years old will yield 1/(2+1)
 * or 1/3.
 * 
 * @see FunctionQuery
 * 
 * @version $Id: ReciprocalFloatFunction.java 1065312 2011-01-30 16:08:25Z rmuir
 *          $
 */
public class ReciprocalFloatFunction extends ValueSource {
	protected final ValueSource source;
	protected final float m;
	protected final float a;
	protected final float b;

	/**
	 * f(source) = a/(m*float(source)+b)
	 */
	public ReciprocalFloatFunction(ValueSource source, float m, float a, float b) {
		this.source = source;
		this.m = m;
		this.a = a;
		this.b = b;
	}

	@Override
	public DocValues getValues(Map context, IndexReader reader)
			throws IOException {
		final DocValues vals = source.getValues(context, reader);
		return new DocValues() {
			@Override
			public float floatVal(int doc) {
				return a / (m * vals.floatVal(doc) + b);
			}

			@Override
			public int intVal(int doc) {
				return (int) floatVal(doc);
			}

			@Override
			public long longVal(int doc) {
				return (long) floatVal(doc);
			}

			@Override
			public double doubleVal(int doc) {
				return (double) floatVal(doc);
			}

			@Override
			public String strVal(int doc) {
				return Float.toString(floatVal(doc));
			}

			@Override
			public String toString(int doc) {
				return Float.toString(a) + "/(" + m + "*float("
						+ vals.toString(doc) + ')' + '+' + b + ')';
			}
		};
	}

	@Override
	public void createWeight(Map context, Searcher searcher) throws IOException {
		source.createWeight(context, searcher);
	}

	@Override
	public String description() {
		return Float.toString(a) + "/(" + m + "*float(" + source.description()
				+ ")" + "+" + b + ')';
	}

	@Override
	public int hashCode() {
		int h = Float.floatToIntBits(a) + Float.floatToIntBits(m);
		h ^= (h << 13) | (h >>> 20);
		return h + (Float.floatToIntBits(b)) + source.hashCode();
	}

	@Override
	public boolean equals(Object o) {
		if (ReciprocalFloatFunction.class != o.getClass())
			return false;
		ReciprocalFloatFunction other = (ReciprocalFloatFunction) o;
		return this.m == other.m && this.a == other.a && this.b == other.b
				&& this.source.equals(other.source);
	}
}
